We used polymerase chain reaction (PCR) to construct recombinants between related viruses in order to investigate structure-function relationships of viral genes. In collaboration with Stephen Feinstone (CBER/FDA), the VP4 gene of polio virus was substituted for the VP4 gene of hepatitis A virus (HAV). The resultant virus was non-infectious, suggesting virus-specific interactions between VP4 and other HAV genes. In collaboration with Kuan Teh Jeang, the SV40 enhancer was substituted for the HIV enhancer in an HIV-CAT construct. The resultant construct was only partially responsive to tat, implicating the HIV enhancer in the tat regulatory system. In collaboration with Roy Repaske, a chimeric MoMuLV-HIV reverse transcriptase (RT) gene was constructed. This gene has HIV-like rather than MoMuLV-like reverse transcriptase activity in bacterial cells, localizing these traits to the HIV substituted segment. With Orit Nahor I constructed a chimeric MoMuLV enhancer in mouse cells. Recently, another MoMuLV chimeria was constructed in which the HIV tat gene, plus an upstream intron, was inserted into the MoMuLV U3 region. The chimeric virus expressed tat, but was noninfectious, while the same chimera with the insert in the opposite orientation did not express tat and was infectious. This chimera will be useful to study effects of splicing on viral infectivity.